The present invention relates to selected planarizing liquids for chemical-mechanical planarization of microelectronic substrates.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) are used in the manufacturing of microelectronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrates and substrate assemblies. FIG. 1 schematically illustrates a CMP machine 10 having a platen 20. The platen 20 supports a planarizing medium 40 that can include a polishing pad 41 having a planarizing surface 42 on which a planarizing liquid 43 is disposed. The polishing pad 41 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed-abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 43 may be a conventional CMP slurry with abrasive particles and chemicals that remove material from the wafer, or the planarizing liquid may be a planarizing solution without abrasive particles. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, and planarizing solutions without abrasive particles are used on fixed abrasive polishing pads.
The CMP machine 10 can also include an underpad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 41. A drive assembly 26 rotates the platen 20 (as indicated by arrow A), and/or it reciprocates the platen 20 back and forth (as indicated by arrow B). Because the polishing pad 41 is attached to the underpad 25, the polishing pad 41 moves with the platen 20.
A wafer carrier 30 is positioned adjacent the polishing pad 41 and has a lower surface 32 to which a substrate 12 may be attached via suction. Alternatively, the substrate 12 may be attached to a resilient pad 34 positioned between the substrate 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 33 may be attached to the wafer carrier to impart axial and/or rotational motion (as indicated by arrows C and D, respectively).
To planarize the substrate 12 with the CMP machine 10, the wafer carrier 30 presses the substrate 12 face-downward against the polishing pad 41. While the face of the substrate 12 presses against the polishing pad 41, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the substrate 12 across the planarizing surface 42. As the face of the substrate 12 moves across the planarizing surface 42, material is continuously removed from the face of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates develop large xe2x80x9cstep heightsxe2x80x9d that create a highly topographic surface across the substrate. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several stages of processing the substrate because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features. For example, it is difficult to accurately focus photo-patterns to within tolerances approaching 0.1 xcexcm on non-uniform substrate surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
In the competitive semiconductor industry, it is also highly desirable to have a high yield in CMP processes by producing a uniformly planar surface at a desired endpoint on a substrate as quickly as possible. For example, when a conductive layer on a substrate is under-planarized in the formation of contacts or interconnects, many of these components may not be electrically isolated from one another because undesirable portions of the conductive layer may remain on the substrate over a dielectric layer. Additionally, when a substrate is over-planarized, components below the desired endpoint may be damaged or completely destroyed. Thus, to provide a high yield of operable microelectronic devices, CMP processing should quickly remove material until the desired endpoint is reached.
The planarity of the finished substrate and the yield of CMP processing is a function of several factors, one of which is the rate at which material is removed from the substrate (the xe2x80x9cpolishing ratexe2x80x9d). Although it is desirable to have a high polishing rate to reduce the duration of each planarizing cycle, the polishing rate should be uniform across the substrate to produce a uniformly planar surface. The polishing rate should also be consistent to accurately endpoint CMP processing at a desired elevation in the substrate. The polishing rate, therefore, should be controlled to provide accurate, reproducible results.
In certain applications, the polishing rate depends on the chemical interaction between the substrate and the planarizing liquid. For example, the polishing rate can depend on the rate at which material at the surface of the substrate is hydrolyzed. The rate at which the hydrolysis reaction proceeds can be dependent on several factors, including the pH of the planarizing liquid adjacent to the substrate. In some CMP operations, the pH of the liquid can vary as the planarization process proceeds. For example, the pH can decrease as material from the substrate and the polishing pad is released into the planarizing liquid. As the pH level decreases, the polishing rate can also decrease because the rate at which the hydrolysis reaction proceeds can decrease. Furthermore, as the hydrolysis reaction rate decreases, the mechanical interaction between the polishing pad and the substrate can dominate the chemical interaction and can increase the likelihood for forming scratches in the surface of the substrate.
Another factor affecting the overall planarity of the substrate assembly is the wetted surface area of the polishing pad. If the polishing pad develops localized dry spots, the polishing pad can be more likely to scratch the substrate because the dry spots are less chemically active than the wetted regions, and therefore the mechanical interaction between the polishing pad and the substrate can dominate the chemical interaction at the dry spots, as discussed above.
One conventional approach to maintaining the pH of the planarizing liquid is to planarize a metal-containing substrate with a conventional polishing pad without fixed-abrasive particles in combination with an acidic or neutral pH slurry containing a suspension of abrasive particles and a chemical buffering agent. However, this approach has several drawbacks. For example, the acidic or neutral pH is not suitable for planarizing substrates containing certain materials, such as oxides. Furthermore, the polishing rate can be influenced by the distribution of the planarizing liquid 43 between the substrate 12 and the planarizing surface 42 of the polishing pad 41. The distribution of the planarizing liquid 43 may not be uniform across the surface of the substrate 12 because the leading edge of the substrate 12 can wipe a significant portion of the planarizing liquid 43 from the polishing pad 41 before the planarizing liquid 43 can contact the other areas of the substrate 12. The nonuniform distribution of planarizing liquid 43 under the substrate 12 can cause certain areas of the substrate 12 to have a higher polishing rate than other areas because they have more contact with the chemicals and/or abrasive particles in the planarizing liquid 43. The surface of the substrate 12 may accordingly not be uniformly planar and in extreme cases, some devices may be damaged or destroyed by CMP processing.
Another approach to the foregoing problem, disclosed in commonly assigned U.S. patent application Ser. No. 09/164,916, assigned to the assignee of the present application, is to provide a fixed abrasive polishing pad with soluble elements that are released into the planarizing liquid as the polishing pad abrades during normal operation. The soluble elements can include surfactants to increase the wetted surface area of the substrate, or a buffering agent to buffer the planarizing liquid. One potential drawback with this approach is that the combination of the planarizing liquid and the chemicals released by the soluble elements may not be compatible with the high-pH environment used to remove materials such as oxides from the substrate 12. Another potential drawback is that the release of the chemicals from the polishing pad may not occur in an entirely uniform fashion, resulting in spatial and/or temporal variations in wetted surface area and/or pH during the planarizing process.
Another drawback with some of the conventional approaches described above is that the frictional forces between the substrate 12 and the polishing pad 41 can become so high that the substrate 12 sticks to the polishing pad 41. For example, in some polishing operations (e.g., xe2x80x9cflat CMPxe2x80x9d), polishing continues after the surface roughness of the substrate 12 has been eliminated to reduce the thickness of the substrate 12. During flat CMP, the frictional forces between the substrate 12 and the polishing pad 41 can increase substantially due to the increase in substrate surface area contacting the polishing pad 41. The substrate 12 can accordingly stick to the polishing pad 41. When the substrate 12 sticks to the polishing pad 41, it can slip out from underneath the carrier 30, causing damage to the substrate 12 and/or the carrier 30. Furthermore, an operator must reinstall the substrate 12 in the carrier 30, increasing the time required to polish the substrate 12. Still further, it can be very difficult to accurately track the total time during which the substrate is planarized, due to the interruption in the planarizing process resulting from reinstalling the substrate 112.
The present invention is directed toward methods and apparatuses for planarizing microelectronic substrates. In one aspect of the invention, the method can include maintaining the pH of a planarizing liquid adjacent a fixed abrasive polishing pad at an approximately constant value. For example, the method can include providing a buffering agent to only a region external to the polishing pad to maintain the pH of the planarizing liquid at an approximately constant alkaline value of between approximately 9 and approximately 13. The buffering agent can be selected to include ammonium hydroxide or potassium hydroxide and at least one of ammonium acetate, ammonium citrate and potassium hydrogen phthalate. Alternatively, the buffering agent can be eliminated and the pH of the planarizing liquid can be selected to have a relatively high value, for example, at least 12.
In another aspect of the invention, the method can include engaging the microelectronic substrate with the planarizing liquid and a fixed abrasive polishing pad, moving at least one of the substrate and the polishing pad relative to the other and controlling a hydrolysis reaction at the surface of the microelectronic substrate or controlling a rate at which the microelectronic substrate scratches by providing a buffering agent to the planarizing liquid while the microelectronic substrate is engaged with the planarizing liquid. For example, removing material from the microelectronic substrate can include removing silicon dioxide from the microelectronic substrate, and controlling the hydrolysis reaction can include promoting a conversion of silicon dioxide to Si(OH)62xe2x88x92.
In still another aspect of the invention, the method can include controlling a drag force between the microelectronic substrate and the polishing pad by selecting the planarizing liquid to include a surfactant. The method can further include removing material from all portions of the surface of the microelectronic substrate at a generally uniform rate and selecting the surfactant to include isopropyl alcohol. The isopropyl alcohol can be about 0.5% to about 2% of the weight of the planarizing liquid. In yet another aspect of this method, the planarizing liquid is a first planarizing liquid and the method further comprises selecting an amount of the surfactant in the first planarizing liquid to reduce a first polishing rate of the first planarizing liquid by no more than about 5% compared to a second polishing rate of a second planarizing liquid not having the surfactant, when the first and second planarizing liquids remove material under generally identical conditions.
A planarizing medium in accordance with still another aspect of the invention can include a fixed abrasive polishing pad and an adjacent planarizing liquid. The planarizing liquid can include at least one of ammonium acetate, polyoxy ethylene ether and isopropyl alcohol for controlling a wetted surface area of the polishing pad. Alternatively, the planarizing liquid can include from about 0.5% to about 2.0% isopropyl alcohol for controlling a friction force between the polishing pad and a microelectronic substrate. The polishing pad can have a generally circular planform shape for mounting to a generally circular platen, or the polishing pad can include an elongated flexible web configured to be wound from a first roller across a platen to a second roller.